1. Field of the Invention
The present invention relates to improvements in a drive s power transfer system of a vehicle, and more particularly to improvements in a structure of an endless chain type power transfer system for a four wheel drive vehicle.
2. Description of the Related Art
Various transfer systems for distributing a drive power of an engine to front wheels and rear wheels have been disclosed and in practical use. Japanese Patent Provisional Publication No. H7-186759 discloses a typical power transfer system where an input shaft and a first output shaft are coaxially arranged in tandem. An end portion of the first output shaft is in but joint with the input shaft, and the other end portion of the first output shaft is supported to a housing through a ball bearing. A drive power from the input shaft to the first output shaft is distributed to a second output shaft through a friction clutch, a first sprocket, an endless chain, and a second sprocket. The distribution ratio of the drive power is determined according to the degree of the engagement of the friction clutch. During the operation of the friction clutch, a thrust load along an axial direction of the first input shaft is applied to the first input shaft and functions to bend the first input shaft. In this condition, a thrust bearing for receiving such thrust load tends to receive a local load such as an edge load. This requires a remarkable increase of the durability of the thrust bearing.